An antenna, also known as an aerial, is an electronic device that converts electric power into radio waves and vice versa. Antennas are used to transmit and/or receive radio frequency (RF) signals. An antenna element is an electrically conductive member of an antenna. Various arrangements of antenna elements are known, such as dipole, monopole, Yagi, spiral and helix, each arrangement having a characteristic radiation pattern, impedance, etc. For example, spiral antennas may be used where wide bandwidths are required. In addition, spiral antennas inherently transmit circularly polarized radio waves and can receive linearly polarized signals, regardless of the linear polarization orientation.
A spiral antenna may include a flat ground plane or a lossy cavity behind, and spaced apart from, its spiral element(s) to change the antenna's radiation pattern to be unidirectional. The ground plane reflects signals back toward the spiral elements. If the ground plane is spaced one-quarter of a wavelength from the driven spiral elements, the reflected signal constructively interferes with signals radiated by the driven spiral elements, providing gain. However, this gain is theoretically realized for only a single frequency, whose wavelength is used to determine the spacing, as well as harmonics of the frequency. Practically, the gain may be realized over a relatively small range of frequencies.
A conical ground plane spaced apart from a planar driven element has different spacings at different radial distances from the center of the ground plane, thereby providing a range of spacings. This range can encompass one-quarter wavelengths for a range of frequencies, thereby providing gain over the range of frequencies. (Caswell, Eric D., “Design and Analysis of Star Spiral with Application to Wideband Arrays with Variable Element Sizes,” Dec. 14, 2001, citing Drewniak, J., et al., “A log-spiral, radiating-line antenna,” 1986.)
Driven elements in some spiral antennas are corrugated in a z direction, rather than flat. For example, U.S. patent application Ser. No. 14/221,467, “Periodic Spiral Antenna,” filed Mar. 21, 2014 by O'Brien, et al. (referred to hereinafter as “O'Brien”) and assigned to the assignee of the present application, describes a cavity-backed spiral antenna with periodic sinusoidally corrugated driven elements. O'Brien's cavity has a flat bottom. O'Brien notes that sinusoidal valleys of the driven elements would be close to the top edge of a wall of a conventional cavity, thereby causing power loss to the grounded cavity. O'Brien's cavity wall has a sinusoidal top edge, shaped the same as the driven element at the outer edge of the spiral, to reduce this power loss.